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Hydrostatic pressure-depth plot construction (view source)
Revision as of 21:18, 29 January 2014
, 21:18, 29 January 2014Initial import
{{publication
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Critical elements of the petroleum system
| chapter = Formation fluid pressure and its application
| frompg = 5-1
| topg = 5-64
| author = Edward A. Beaumont, Forrest Fiedler
| link = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
The goal of constructing a hydrostatic pressure–depth plot is to identify pressures greater than the hydrostatic gradient that may correspond to a hydrocarbon-bearing zone. A hydrostatic pressure–depth plot can be constructed from any of the following:
* Measured pressures
* Regional rule-of-thumb pressure gradients
* Pressures calculated from water density
Calculated pressures are much less accurate than measured pressures but can be used with some effectiveness when they are supplemented with other petrophysical data.
==Procedure: constructing a plot==
The table below outlines a procedure for constructing a hydrostatic pressure-depth plot for a single well.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Using graph paper with a linear grid, label the X-axis as pressure and the Y-axis as depth. Use as large a scale as possible. Also, make the Y-axis on at least one plot the same scale as on the well logs to aid in interpretation.
|-
| 2
| Plot measured pressure from the aquifers (100% S<sub>w</sub> ) in the well. If none is available, go to step 3.
|-
| 3
| Plot measured pressure from the aquifers in nearby wells. If none is available, go to step 4.
|-
| 4
| Calculate and plot hydrostatic pressures for a depth above and a depth below the zone of interest. Use the rule-of-thumb pressure gradient for that zone. If it is not available, calculate the gradient from the water density using density measured from the formation water or calculated from R<sub>w</sub> . For help, see the following sections.
|}
==Calculating pressure gradients from water density==
The formation fluid pressure at any depth in a well is a function of the average formation water density (ave. ρ<sub>w</sub>) above that depth, not the density of the formation water at any particular depth. Formation water generally becomes more dense with increasing depth.
To calculate water pressure gradient (P<sub>grad</sub>), use the following formula:
:<math>\mbox{P}_{\rm grad} = \mbox{ave. } \rho_{\rm w} \times 0.433 \mbox{ psi/ft}\end{align*}where:\begin{align*}\mbox{ave. } \rho_{\rm w} = \frac{\Sigma_{1}^{\rm n}\rho_{\rm w}}{\mbox{n}}</math>
:<math>\rho_{\rm w} &= \mbox{water density}</math>
where:
* ave.ρ<sub>w</sub> = <math>\frac{\Sigma_{1}^{\rm n}\rho_{\rm w}}{\mbox{n}}</math>
* ρ<sub>w</sub> = water density
For example, given ave. ρ = 1.13, the equation works as follows:
:<math>\mbox{P}_{\rm grad} = 1.13 \times 0.433 \mbox{ psi/ft} = 0.489 \mbox{ psi/ft}</math>
==Table of water pressure gradients==
The table below lists hydrostatic pressure gradients, water density, and salinity in weight percent total dissolved solids (TDS).
{| class = "wikitable"
|-
! Gradient (psi/ft)
! Density (g/cc)
! TDS (ppm)
! TDS (wt %)
|-
| 04.33
| 1.000
|
| 0
|-
| 0.437
| 1.010
| 13,500
| 13.5
|-
| 0.441
| 1.020
| 27,500
| 27.5
|-
| 0.444
| 1.029
| 37,000
| 37.0
|-
| 0.445
| 1.030
| 41,400
| 41.4
|-
| 0.451
| 1.040
| 55,400
| 55.4
|-
| 0.454
| 1.050
| 69,400
| 69.4
|-
| 0.459
| 1.060
| 83,700
| 83.7
|-
| 0.463
| 1.070
| 98,400
| 98.4
|-
| 0.465
| 1.075
| 100,000
| 100.0
|-
| 0.467
| 1.080
| 113,200
| 113.2
|-
| 0.471
| 1.090
| 128,300
| 128.3
|-
| 0.476
| 1.100
| 143,500
| 143.5
|-
| 0.480
| 1.110
| 159,500
| 159.5
|-
| 0.485
| 1.120
| 175,800
| 175.8
|-
| 0.489
| 1.130
| 192,400
| 192.4
|-
| 0.491
| 1.135
| 200,000
| 200.0
|-
| 0.493
| 1.137
| 210,000
| 210.0
|-
| 0.500
| 1.153
| 230,000
| 230.0
|-
| 0.510
| 1.176
| 260,000
| 260.0
|}
==Rules of thumb==
Most sedimentary basins have a rule of thumb for average hydrostatic water pressure gradients. For the Gulf Coast basin, it is 0.465 psi/ft. For Rocky Mountain basins, it is 0.45 psi/ft. For fresh water, it is 0.433 psi/ft. If measured hydrostatic pressure is not available for a well, find out the accepted rule-of-thumb average hydrostatic pressure gradient for the depth of the zone of interest where the well is located.
==Example of TDS vs. depth==
Water density is a function of its TDS concentration. The hydrostatic pressure at any depth is a function of TDS concentration from the surface to that point. The plot below of TDS vs. depth is from southern Arkansas. It shows a gradual increase in TDS from the surface to about [[depth::2000 ft]], probably due to meteoric effects, and then a linear, more rapid increase in TDS from 2000 to [[depth::10,000 ft]]. Generally, below the depth of meteoric water influence, the increase in TDS in connate brines is linear and ranges from 25,000 to 100,000 mg/1 per [[depth::1000 ft]] (80 to 300 mg/1 per m).<ref name=ch05r6>Dickey, P., A., 1969, Increasing concentration of subsurface brines with depth: Chemical Geology, vol. 4, p. 361–370., 10., 1016/0009-2541(69)90055-2</ref> There are exceptions to this general case.
Such consistent salinity increase with depth is not unique to the East Texas basin but is characteristic of most basins.
[[file:formation-fluid-pressure-and-its-application_fig5-4.png|thumb|{{figure number|5-4}}. Copyright: Dickey, 1969; courtesy Chemical Geology.]]
==See also==
* [[Determining hydrostatic pressure gradient]]
* [[Estimating formation water density]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Formation fluid pressure and its application]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Critical elements of the petroleum system
| chapter = Formation fluid pressure and its application
| frompg = 5-1
| topg = 5-64
| author = Edward A. Beaumont, Forrest Fiedler
| link = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
The goal of constructing a hydrostatic pressure–depth plot is to identify pressures greater than the hydrostatic gradient that may correspond to a hydrocarbon-bearing zone. A hydrostatic pressure–depth plot can be constructed from any of the following:
* Measured pressures
* Regional rule-of-thumb pressure gradients
* Pressures calculated from water density
Calculated pressures are much less accurate than measured pressures but can be used with some effectiveness when they are supplemented with other petrophysical data.
==Procedure: constructing a plot==
The table below outlines a procedure for constructing a hydrostatic pressure-depth plot for a single well.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Using graph paper with a linear grid, label the X-axis as pressure and the Y-axis as depth. Use as large a scale as possible. Also, make the Y-axis on at least one plot the same scale as on the well logs to aid in interpretation.
|-
| 2
| Plot measured pressure from the aquifers (100% S<sub>w</sub> ) in the well. If none is available, go to step 3.
|-
| 3
| Plot measured pressure from the aquifers in nearby wells. If none is available, go to step 4.
|-
| 4
| Calculate and plot hydrostatic pressures for a depth above and a depth below the zone of interest. Use the rule-of-thumb pressure gradient for that zone. If it is not available, calculate the gradient from the water density using density measured from the formation water or calculated from R<sub>w</sub> . For help, see the following sections.
|}
==Calculating pressure gradients from water density==
The formation fluid pressure at any depth in a well is a function of the average formation water density (ave. ρ<sub>w</sub>) above that depth, not the density of the formation water at any particular depth. Formation water generally becomes more dense with increasing depth.
To calculate water pressure gradient (P<sub>grad</sub>), use the following formula:
:<math>\mbox{P}_{\rm grad} = \mbox{ave. } \rho_{\rm w} \times 0.433 \mbox{ psi/ft}\end{align*}where:\begin{align*}\mbox{ave. } \rho_{\rm w} = \frac{\Sigma_{1}^{\rm n}\rho_{\rm w}}{\mbox{n}}</math>
:<math>\rho_{\rm w} &= \mbox{water density}</math>
where:
* ave.ρ<sub>w</sub> = <math>\frac{\Sigma_{1}^{\rm n}\rho_{\rm w}}{\mbox{n}}</math>
* ρ<sub>w</sub> = water density
For example, given ave. ρ = 1.13, the equation works as follows:
:<math>\mbox{P}_{\rm grad} = 1.13 \times 0.433 \mbox{ psi/ft} = 0.489 \mbox{ psi/ft}</math>
==Table of water pressure gradients==
The table below lists hydrostatic pressure gradients, water density, and salinity in weight percent total dissolved solids (TDS).
{| class = "wikitable"
|-
! Gradient (psi/ft)
! Density (g/cc)
! TDS (ppm)
! TDS (wt %)
|-
| 04.33
| 1.000
|
| 0
|-
| 0.437
| 1.010
| 13,500
| 13.5
|-
| 0.441
| 1.020
| 27,500
| 27.5
|-
| 0.444
| 1.029
| 37,000
| 37.0
|-
| 0.445
| 1.030
| 41,400
| 41.4
|-
| 0.451
| 1.040
| 55,400
| 55.4
|-
| 0.454
| 1.050
| 69,400
| 69.4
|-
| 0.459
| 1.060
| 83,700
| 83.7
|-
| 0.463
| 1.070
| 98,400
| 98.4
|-
| 0.465
| 1.075
| 100,000
| 100.0
|-
| 0.467
| 1.080
| 113,200
| 113.2
|-
| 0.471
| 1.090
| 128,300
| 128.3
|-
| 0.476
| 1.100
| 143,500
| 143.5
|-
| 0.480
| 1.110
| 159,500
| 159.5
|-
| 0.485
| 1.120
| 175,800
| 175.8
|-
| 0.489
| 1.130
| 192,400
| 192.4
|-
| 0.491
| 1.135
| 200,000
| 200.0
|-
| 0.493
| 1.137
| 210,000
| 210.0
|-
| 0.500
| 1.153
| 230,000
| 230.0
|-
| 0.510
| 1.176
| 260,000
| 260.0
|}
==Rules of thumb==
Most sedimentary basins have a rule of thumb for average hydrostatic water pressure gradients. For the Gulf Coast basin, it is 0.465 psi/ft. For Rocky Mountain basins, it is 0.45 psi/ft. For fresh water, it is 0.433 psi/ft. If measured hydrostatic pressure is not available for a well, find out the accepted rule-of-thumb average hydrostatic pressure gradient for the depth of the zone of interest where the well is located.
==Example of TDS vs. depth==
Water density is a function of its TDS concentration. The hydrostatic pressure at any depth is a function of TDS concentration from the surface to that point. The plot below of TDS vs. depth is from southern Arkansas. It shows a gradual increase in TDS from the surface to about [[depth::2000 ft]], probably due to meteoric effects, and then a linear, more rapid increase in TDS from 2000 to [[depth::10,000 ft]]. Generally, below the depth of meteoric water influence, the increase in TDS in connate brines is linear and ranges from 25,000 to 100,000 mg/1 per [[depth::1000 ft]] (80 to 300 mg/1 per m).<ref name=ch05r6>Dickey, P., A., 1969, Increasing concentration of subsurface brines with depth: Chemical Geology, vol. 4, p. 361–370., 10., 1016/0009-2541(69)90055-2</ref> There are exceptions to this general case.
Such consistent salinity increase with depth is not unique to the East Texas basin but is characteristic of most basins.
[[file:formation-fluid-pressure-and-its-application_fig5-4.png|thumb|{{figure number|5-4}}. Copyright: Dickey, 1969; courtesy Chemical Geology.]]
==See also==
* [[Determining hydrostatic pressure gradient]]
* [[Estimating formation water density]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Formation fluid pressure and its application]]